Thermal efficiency and emissions of a commercial vehicle hybrid-dedicated diesel engine with Miller/Atkinson cycle and high compression ratio: An experimental study

To address energy security and global environmental challenges, this study conducted bench tests to enhance the thermal efficiency of commercial vehicle hybrid-dedicated diesel engines, developing powertrain systems with high fuel economy and competitive total cost of ownership. Focusing on key conditions for thermal efficiency optimization, this study employed the Miller/Atkinson cycle and high compression ratio configurations. Through experimental and thermodynamic analyses, the study investigated how effective compression ratio (ECR) influences combustion processes and emission characteristics, proposing a technical route to improve thermal efficiency under limited peak cylinder pressure. Experimental results demonstrated that ECR modifications effectively regulate the in-cylinder thermal environment, with reduced temperature and pressure providing margin for advancing the start of injection (SOI), thereby minimizing combustion losses. Without a significant increase in NOx emissions relative to the baseline, the late intake valve closing (LIVC) strategy, combined with a high geometric compression ratio (GCR), significantly reduced soot emissions at low load. Compared to the baseline, large soot particles are reduced by over 85%. The increased expansion ratio improved thermal-work conversion efficiency while reducing pumping losses, resulting in an approximately 1.2% increase in the maximum theoretical thermal efficiency. The maximum brake thermal efficiency of the optimized diesel engine increased by 2.5%, and the minimum brake specific fuel consumption (BSFC) dropped from 192.6 [g/(kW·h)] to 187.5 [g/(kW·h)]. Furthermore, the BSFC under low-load conditions was significantly reduced by over 10 [g/(kW·h)].